Air traffic control (ATC) centers are used at most airports to coordinate take-offs, landings, and general aircraft traffic around the airport. Traditionally, a pilot uses a radio to speak to an ATC center to request permission or to receive instructions therefrom. With increasing air traffic it has become difficult for ATC centers to process all of the oral communications from aircraft. Consequently, datalink applications have been developed to provide textual communication between pilots and air traffic controllers.
An exemplary datalink application is the Aircraft Communications, Addressing, and Reporting System (ACARS), which is a two-way data communications system used for transmission of text messages between aircraft and ground stations via radio or satellite. A complete datalink communication, which may be generated either manually or automatically, is referred to as a datalink message. Messages from the aircraft to the ground are referred to as downlink messages and messages from the ground to the aircraft are referred to as uplink messages. Services available via datalink include flight plans, weather reports and forecasts, air traffic services, and two-way messaging.
Another datalink application is the Controller Pilot Data Link Communication (CPDLC), which provides for the direct exchange of text messages between an air traffic controller and a pilot. The CPDLC system enables the pilot to communicate electronically with an ATC center by guiding the pilot through a series of screen configurations or displays that either elicit flight information from the pilot or notify the pilot regarding flight information. The CPDLC system is used to send information between aircraft and air traffic control more safely and efficiently.
The time it takes for a datalink message to transit a network varies with the load (i.e, message volume) on the network. Some applications such as CPDLC require a specific level of performance. If the CPDLC messages are delayed too long (e.g., 30 seconds), the messages become stale. Other high priority messages, such as air traffic control messages and wind shear warnings, may also be very time critical. If such messages are delayed because of high network traffic, serious safety issues can be raised resulting in potentially dangerous situations for an aircraft.